JPH11230870A - Nitrogen oxide detecting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To implement a compact, lightweight, and inexpensive device suitable for continuously detecting and measuring nitrogen oxide over a long period of time with little need for time or energy for maintenance, and therefore without attendance in regard to a nitrogen oxide detecting device. SOLUTION: A nitrogen oxide detecting device is provided with a switching valve 4, into which air is directly supplied through the first gas channel 71 , and into which air is supplied through the second gas channel 72 and a column 3 filled with particles (e.g. alumina beads) holding potassium permanganates; and an NO2 gas sensor 5 in an electrochemical system, into which air containing NOx of NO+NO2 or air containing only approximately NO2 discharged from the switching valve 4 through the third gas channel 73 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small and light nitrogen oxide detector suitable for measuring the concentration of nitrogen oxide in air.

[0002] Nitrogen oxides or sulfur oxides are known to be the causative agents of acid rain and photochemical smog, and are also air pollutants that make humans nervous or cause respiratory damage. Therefore, it is important to detect and measure these air pollutants.

[0003] In particular, nitrogen oxides are contained in a large amount in the exhaust gas of automobiles. Therefore, the concentration of the nitrogen oxides varies depending on the number of automobiles. Therefore, the state of environmental pollution can be known by detecting the concentration. be able to.

[0004] However, since the current detection device is large and expensive, there is a problem in gathering a lot of information by installing it in a large number of bases, and the present invention solves the problem. Means are disclosed.

[0005]

2. Description of the Related Art In general, a chemiluminescence method is known as a representative and mainstream measuring means for measuring the concentration of nitrogen oxides in the air, and the following equations (1) and (2) implement the method. It is a formula showing the chemical reaction in the case of doing.

NO + O ₃ → NO ₂ ^* + O ₂ (1) NO ₂ ^* → NO ₂ + hν (2)

In the chemical reaction luminescence method, NO and ozone are reacted, and as shown in equation (1), NO in an excited state
To generate ₂ ^* and O _2, the NO ₂ ^* is a method of measuring the amount of fluorescence generated when returning to the ground state as seen in equation (2), NO ₂ is reduced to a pre NO It can be measured by keeping it.

[0008] As can be seen from the above, in order to carry out the chemical reaction luminescence method, an ozone generator is required, and a certain concentration of oxygen is required to generate a certain concentration of ozone. In order to remove water in the gas that generates ozone, for example, a column containing silica gel is required, and when measuring NO ₂ , a catalyst for reducing to NO and a temperature of 300 ° C. Therefore, a heating device for heating is required.

As described above, in the chemical reaction luminescence method, it is essential to use ozone, and this causes another problem.

For example, since ozone is a harmful substance, ozone that did not contribute to the reaction shown in the formula (1) cannot be released into the atmosphere as it is, and therefore, a device for detoxifying ozone, For example, a device for reducing oxygen is required.

Further, in the chemiluminescence method, it is necessary to cause many chemical reactions, so that the number of parts used in the method is increased, and the detecting device is inevitably expensive, and in addition, complicated gas flow is required. Since a road is necessary, it is unavoidable that the size is increased. For example, 400 [mm] × 50
It is also 0 [mm] × 200 [mm].

[0012] In addition, maintenance such as replacement of chemicals must be performed frequently, and therefore, it is not suitable for installation outdoors to perform unattended and continuous detection and measurement.

[0013]

The present invention is small, lightweight, inexpensive and requires little maintenance, and is therefore suitable for detecting and measuring nitrogen oxides continuously for a long time in an unmanned state. To realize a simple device.

[0014]

According to the present invention, NO
Is oxidized to NO ₂ , and an electrochemical method (refer to JP-A-7-234203, if necessary) is used to determine the concentrations of NO and NO ₂ and the total concentration of NO _x , It is based on the use of zero gas generator technology, a device that creates a standard concentration of low concentration of pollutants. In the following description, NO, NO ₂ ,
The concentrations of NO _{x and the} like are set to [NO], [NO ₂ ], [NO _x ]
Will be represented by

[0015] The zero gas generator is a device for obtaining clean air by removing all traces of contaminants contained in ordinary air, and removing nitrogen oxides used in the device. As a measure, potassium permanganate is converted into alumina beads having many fine voids, that is, activated alumina.
Those impregnated beads leave packed in a column, by flowing the air into the column, since NO in air and changes into oxidized NO _2, and the NO ₂ to be adsorbed by carbon black ing.

From the above description, in the nitrogen oxide detecting apparatus according to the present invention, (1) air is directly fed through a _first gas flow path (for example, the first gas flow path 7 ₁ ). It is and air second gas flow path (eg, the second gas flow path 7 ₂₎
And a switching valve (for example, switching valve 4) that is fed through a column (for example, column 3) packed with particles (for example, alumina beads) holding potassium permanganate
And N which is NO + NO ₂ sent out from the switching valve.
An electrochemical NO ₂ gas sensor (for example, a constant type) in which air containing O _x or air containing substantially only NO ₂ flows through a _third gas flow path (for example, the third gas flow path 7 ₃ ) and is supplied thereto. A potential electrolysis type NO ₂ gas sensor 5), or

(2) In the above (1), the electrochemical NO ₂ gas sensor is either a constant potential electrolysis type or an electromotive force measurement type, or

(3) In the above (1) or (2),
It is characterized by comprising a pump (for example, a pump 1) installed at an appropriate position in a gas flow path for flowing air and a gas flow meter (for example, a gas flow meter 2) for detecting the flow rate of air and controlling the operation of the pump. I do.

By adopting the above-mentioned means, it is possible to realize a nitrogen oxide detector which is smaller, cheaper and easier to maintain than a nitrogen oxide detector which performs a chemiluminescence method. It is possible to perform unattended and continuous detection and measurement.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a main block diagram showing a nitrogen oxide detecting device for explaining an embodiment of the present invention, in which 1 is a pump, and 2 is a gas flow meter. 3, column, 4 switching valve, 5 NO ₂ gas sensor, 6
Indicates a gas inlet pipe, 7 ₁ , 7 ₂ , and 7 ₃ indicate first to third gas flow paths, and 8 indicates a gas delivery pipe.

The pump 1 is fed back with a flow rate signal from the gas flow meter 2 and its operation is controlled by the signal. Note that the pump 1 and the gas flow meter 2
_{(2) It} may be installed in the gas delivery pipe 8 on the delivery side of the gas sensor 5.

Column 3 is packed with alumina particles impregnated with potassium permanganate.

FIG. 2 is a cutaway side view of a main part specifically showing the column described with reference to FIG. 1. In the figure, 11 is a plastic container, 12 is an air supply port, 13 is an exhaust port, 1
4A and 14B are stainless steel meshes, 15A and 1
5B shows a breathable sponge, and 16 shows alumina beads impregnated with potassium permanganate.

FIG. 3 is a view for explaining an apparatus used for impregnating potassium permanganate into alumina beads. 21 is a desiccator, 22 is a cover plate, 23 and 24 are cocks, 25 is a vacuum pump, 26 Is a pipette, 27 is a cock, 28 is a beaker, 29 is activated alumina beads, 3
0 indicates water or an aqueous solution of potassium permanganate, respectively.

An example of a process for preparing the alumina beads 16 impregnated with potassium permanganate using the illustrated apparatus will be described.

A commercially available diameter of 4 mm to 8 mm
Alumina beads in a beaker 28 of, for example, 300 g
It is stored and set in the desiccator 21 which can be decompressed. A 300 [ml] pipette 26 filled with water is set in a hole provided in the lid plate 22 of the desiccator 21,
The tip is located immediately above the beaker 28. The cocks 24 and 27 are closed, the decompression pump 25 is connected to the cock 23 attached to the desiccator 21, and
After reducing the pressure to about 1/20 [atm], the cock 23 is closed. The cock 27 is opened, water is allowed to flow until the alumina beads 29 are sufficiently wet, and then the cock 27 is closed. The cock 24 is gradually opened to return the interior of the desiccator 21 to normal pressure. The beaker 28 is taken out, excess water is allowed to flow out, and the weight is measured to determine the weight of the water impregnated in the alumina beads 29. A saturated aqueous solution of potassium permanganate in the same amount as the weight of the water impregnated in the alumina beads 29 measured in step (1) is placed in the pipette 26. Through the same steps as the steps described above,
Alumina beads 29 are impregnated with a saturated aqueous solution of potassium permanganate. However, use the entire potassium permanganate saturated aqueous solution. The beaker 28 is taken out of the desiccator 21, set in a dryer, and dried at a temperature of, for example, 110 ° C. for 5 hours.

The weight of the alumina beads 29 obtained as described above was 321 g, and the weight of the impregnated potassium permanganate was about 21 g. Is filled in the plastic container 11 described with reference to FIG. 2, and the plastic container 11 is made of, for example, a transparent acrylic material, has a diameter of 40 mm, and a length of 100 mm.
[Mm].

The switching valve 4 is a so-called electromagnetic valve which is driven by an electromagnetic force. The switching valve 4 sends air supplied from the pump 1 directly to the NO ₂ gas sensor 5 or connects the column 3 to the NO ₂ gas sensor 5. It serves to switch the passed air to be sent to the NO ₂ gas sensor 5.

The NO ₂ gas sensor 5 may be of any type as long as it is of the electrochemical type, but in the case of the present invention, the constant-potential electrolytic type (if necessary, No. 61-56777 ", and Japanese Patent Publication No. 6-5637.
6) and an electromotive force measurement type (refer to Japanese Patent Application Laid-Open No. 4-142455 if necessary). In addition, NO ₂ gas of constant potential electrolysis type
A commercially available sensor was used.

FIGS. 4A and 4B are main part explanatory diagrams showing an electrochemical NO ₂ gas sensor, in which FIG. 4A shows a constant potential electrolysis type, and FIG. 4B shows an electromotive force measurement type. .

In FIG. 4A, reference numeral 31 denotes an electrolyte;
Denotes a working electrode, 33 denotes a counter electrode, 34 denotes a reference electrode, 35 denotes a diffusion film, and 36 denotes an air flow passage. It should be noted that the drawing shows a sensor head portion.

The constant potential electrolysis method is a method of measuring a current I flowing under an applied voltage V = constant condition, while measuring a surface potential E of a working electrode 32 with a reference electrode 34.
Voltage V between counter electrode 33 based on surface potential E
Is controlled to be constant, and the current I flowing at that time is measured. The substance to be measured has a steady concentration gradient in the diffusion film 35, and the gradient of the gradient is proportional to the concentration of the substance, and the current I flows in proportion to the concentration on the electrode surface.

When NO ₂ is measured by the above-mentioned potentiostatic NO ₂ gas sensor, the reaction at the working electrode 32 (cathode) is represented by the following equation (1), and the counter electrode 33 (anode)
Is represented by the following equation (2), and therefore, the entire reaction equation is represented by the following equation (3). ^{_{NO 2 + 2e - → NO +}} O 2- ···· (1) O 2- → 1 / 2O 2 + 2e - ···· (2) NO 2 → NO + 1 / 2O 2 ···· (3)

The size of the sensor head shown is 30
[Mm] phi × 20 [mm ^2] to 100 [mm] phi × 30
[Mm ² ], and the overall size is 150 × 1
It is about 00 × 50 [mm ³ ].

In FIG. 4B, 41 is a solid electrolyte (NASICON disk), 42 is an auxiliary layer, 43 is a mesh made of Au, 44 is a wire made of Au, 45 is an inorganic adhesive layer, and 46 is platinum black. , 47 indicate a mesh made of Pt, and 48 indicates a quartz tube. In the figure,
The part of the sensor head is shown.

The electromotive force measurement method is a method for measuring a voltage generated by forming a battery.
It is common to use a solid electrolyte that can pass ions,
The solid electrolyte used here is NASICON (Na ⁺
This is called a super ionic conductor.

[0037] While the conventional NASICON electromotive force is small, although the to form an auxiliary layer 42 made of NaNO ₂ -Li ₂ CO ₃ on the surface of the electromotive force is increased, CO ₂ in air (0.03 [% By volume]).

When the molar ratio of NaNO ₂ —Li ₂ CO ₃ is 9:
If 1, the influence of CO ₂ is decreased, need only generate about 3 [ppbv] about noise against the concentration of NO _2, moreover, particularly good with respect to NO ₂ extremely low concentration sensitive , And NO of 5 [ppbv] or more
_{2 The} concentration can be measured.

The electromotive force in the NO ₂ gas sensor of the electromotive force measuring method is 5 [ppb] at 150 ° C.
One-electron reduction of NO ₂ was estimated from the slope according to Nernst's equation over a wide NO ₂ concentration range of 200 ppm. In addition, 10 [pp] at 150 [° C]
m] and 5 [ppb] are about 8
[Seconds] and 3 [minutes]. Since the sensitivity of the NO ₂ gas sensor is low unless the temperature is raised to a high temperature of about 150 ° C., it is common to use a small heater around the heater while heating.

When NO ₂ is measured by the NO ₂ gas sensor of the electromotive force measuring method, the reaction at the Au mesh 43 as the working electrode is expressed by the following equation (4), and the Pt mesh as the counter electrode is The reaction in 47 is represented by the following formula (5), and therefore, the entire reaction formula is as shown in the following formula (6). Na ⁺ + NO ₂ + e ⁻ = NaNO ₂ (4) Na ⁺ + ／ O ₂ + e ⁻ = ｅNa ₂ O (in NASICON) (5) NO ₂ + ／ Na ₂ O = NaNO ₂ + ／ O ₂ ··· (6)

The size of the sensor head shown is 15
[Mm] φ × 20 [mm ³ ], and the overall size is about 100 × 100 × 30 ^t [mm ² ].

Based on the description of the structure of the nitrogen oxide detection device, a case where the concentration of nitrogen oxides in the air is measured using the nitrogen oxide detection device will be described below. Experimentally produced air containing fixed concentrations of NO and NO ₂ .

Column 3 in the nitrogen oxide detector is
As described above, like the one used in the zero gas generator, it is filled with alumina beads impregnated with potassium permanganate, and the NO ₂ gas sensor 5 is of a potentiostatic electrolytic type.

[0044] When fed the air from the pump 1, via the column 3 together with a portion reaches the switching valve 4 flows a first gas flow path _61, the remainder flows through the second gas flow passage 6 ₂ To reach the switching valve 4.

Here, focusing on the nitrogen oxides, the air directly reaching the switching valve 4 contains NO + NO _2, and the air passing through the column 3 contains only NO _2. By switching to either, N
In the O ₂ gas sensor 5, NO + NO ₂ , that is, NO _x
Or only NO ₂ is detected.

Therefore, [NO] + [NO ₂ ] = [N
O _x ] and [NO ₂ ] can be measured,
[NO] from the calculation of [NO _x ] − [NO ₂ ] = [NO]
You can know.

FIG. 5 is a table showing a measurement example of the nitrogen oxide concentration obtained by an experiment in the present invention. FIG. 5 shows three examples, and in each case, It can be seen that accurate measurements were obtained, at least 10 [pp
b] It will be understood that [NO] and [NO ₂ ] of the order and the total [NO _x ] thereof can be measured.

FIG. 6 is a table showing the advantages and disadvantages of an example of the nitrogen oxide detector of the present invention and the nitrogen oxide detector of the chemiluminescence method. in the apparatus, not only the detection portion is small, the supply and concentration measurement becomes the reference O _2, and is unnecessary O ₃ generator and remover is therefore since the flow path of the air is also simple, the number of tubes It can be seen that the size can be reduced as a whole because the number of pumps and solenoid valves is small and the total length is short.

The nitrogen oxide detecting apparatus according to the present invention has a great advantage in that nitrogen oxide can be continuously detected and measured over a long period of time in an unmanned state. N with potassium permanganate impregnated
Since the means for oxidizing O to NO ₂ is employed, the oxidizing ability is naturally lost with the passage of time. Therefore, it is necessary to perform maintenance every predetermined period. It has been found that the period from to maintenance can range from three months to one year.

This period depends not only on the concentration of NO contained in the air but also depends on the size of the column and the amount of the activated alumina beads to be packed.

The size of the detection part is 150 × 200 × 50
^In the case of about ^t , the size of the column is 40 mm
It must be about 100 [mm ³ ], in which case it is necessary to replace it with a new one about every six months.

It is essential to use a gas permeable sponge which is a filter for removing dust and the like in the air. The gas permeable sponge needs to be replaced when clogging occurs. Even if the filter is provided with an automatic cleaning function, the filter must be replaced every three to six months. In addition, in the case of a sensor based on an electrochemical system, it must be replaced every three months. It is desirable to perform calibration.

From the above points, it is preferable to perform maintenance every three months in the sensor of the present invention. However, in the chemiluminescence method, it is necessary to dry and replace silica gel at the start of measurement. It is said that the color of the sensor of the present invention is almost maintenance-free, considering that the color changes in 40 hours depending on the amount, and that the filter replacement is the same. is not.

[0054]

In the nitrogen oxide detector according to the present invention, the first gas flow path through which air can be directly fed to the switching valve and the column filled with particles holding potassium permanganate are used. Second gas flow path through which air can be sent to the switching valve, and NO + NO sent out from the switching valve.
_A third gas flow path is provided for feeding air containing NO _x or air containing substantially only NO ₂ to an electrochemical NO ₂ gas sensor.

By adopting the above configuration, it is possible to realize a nitrogen oxide detecting device that is small, inexpensive, and easy to maintain as compared with a nitrogen oxide detecting device that performs a chemiluminescence method. It is possible to perform unattended and continuous detection and measurement.

[Brief description of the drawings]

FIG. 1 is a main block diagram showing a nitrogen oxide detecting device for describing an embodiment of the present invention.

FIG. 2 is a cutaway side view of a main part specifically showing the column described with reference to FIG. 1;

FIG. 3 is a diagram illustrating an apparatus used when impregnating alumina beads with potassium permanganate.

FIG. 4 is an explanatory view of a principal part showing an electrochemical type NO ₂ gas sensor.

FIG. 5 is a table showing a measurement example of a nitrogen oxide concentration obtained by an experiment in the present invention.

FIG. 6 is a table showing the advantages and disadvantages of an example of the nitrogen oxide detector of the present invention and the nitrogen oxide detector of the chemiluminescence method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Pump 2 Gas flow meter 3 Column 4 Switching valve 5 NO ₂ gas sensor 6 Gas inlet pipe 7 ₁ , 7 ₂ , 7 ₃ First to third gas flow paths 8 Gas outlet pipe

Claims (3)

[Claims] 1. A switch in which air is fed directly through a first gas flow path and air is fed through a second gas flow path and a column filled with particles holding potassium permanganate. A valve, and NO _x that is NO + NO ₂ delivered from the switching valve.
A nitrogen oxide sensor comprising an electrochemical NO ₂ gas sensor through which air containing NO or air containing substantially only NO ₂ flows and is supplied through a third gas flow path. 2. The nitrogen oxide detecting device according to claim 1, wherein the electrochemical NO ₂ gas sensor is one of a potentiostatic electrolytic type and an electromotive force measuring type. 3. The apparatus according to claim 1, further comprising a pump installed at an appropriate position in the gas flow path to flow the air, and a gas flow meter for detecting the flow rate of the air and controlling the operation of the pump. Nitrogen oxide detector.